Optical networks are continuously evolving in order to meet ever-increasing voice and data traffic demands. There is a need for increased multi-channel capacity in wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) systems, increased reliability, and reduced cost. Conventional optical networks can utilize forward error correction (FEC) in order to improve system margins, to lower overall network costs, to improve optical layer operations, administration, and maintenance (OAM), and to improve network reliability.
FEC is a methodology that enables bits that have been received but improperly decoded to be detected and corrected. Each frame of working data includes corresponding error-check data. Among other things, FEC allows a system to utilize a decreased signal-to-noise ratio (SNR), while maintaining a fixed error probability. Advantageously, FEC is able to correct a specific fraction of the errored data completely, regardless of the source of the errors. For example, decreased SNR means that optical amplifiers can be spaced at longer distances, reducing cost, and that a faster signal may be used, for example. A further advantage is optical component performance can be lowered with a corresponding cost decrease, while still maintaining error free data transmission after the FEC. One of such optical impairments can be Polarization Mode Dispersion (PMD), which generally arises in older, lower quality transmission fiber. Disadvantageously, the inclusion of error-check data means that increased bandwidth, as well as increased transmitter and receiver complexity, are required. A further disadvantage is FEC's limited ability to deal with error bursts, i.e. with a long sequence of errored bits.
PMD is an increasing concern for modern fiber-optic networks. The increased requirements for capacity are continually driving channel data rates up from predominantly 2.5 Gb/s several years ago, to more recent 10 Gb/s, to a current transition to 40 Gb/s rates with 100 Gb/s rates being actively studied for next generation systems. PMD results as light travels down a single-mode fiber in two inherent polarization modes. When the core of the fiber is asymmetric, the light traveling along one polarization mode faster or slower than the light traveling along the other polarization mode, resulting in a pulse overlapping with others, or distorting the pulse to such a degree that it is undetectable by a receiver. PMD concerns are compounded in today's high-speed transmission optical networks, since PMD impairment scales linearly with data rate. For example, a 100 Gb/s signal is 10 times more susceptible to PMD than a similarly modulated 10 Gb/s signal. Further, PMD varies dynamically with temperature changes, infinitesimal asymmetries in the fiber core, etc., and impacts diverse wavelength channels differently. Thus, it is a wavelength and time-dependent impairment. The time constant of the PMD impairment is related to the mechanic and thermal perturbations associated with the optical transmission path.
Rapid mechanical changes can result in sub-millisecond changes, while buried fiber-optic cable can be quite stable for months. A typical time constant is in the range of 100 ms to many minutes, with the data errors being produced within the corresponding time periods. Thus, PMD-induced errors tend to be bursty, generally exceed the FEC correction capability, and present a significant challenge. Several recent investigations have shown that a typical fiber-optic transmission system with buried fiber cable segments can be represented as stable on very long time scales and to have a finite PMD value. These PMD-causing segments are separated by small fiber lengths that correspond to optical amplifier locations, and are responsible for fast polarization rotation. This representation is frequently referred to as a “hinge model”, with polarization rotation inducing exposed hinges separated by substantially stable PMD-causing cable segments.
The mitigation of PMD-induced impairments can be divided into two general categories. First, channel-based PMD compensators are used at receivers. This approach requires a single expensive device for each WDM channel. Attempts have been made to reduce the number of required compensators by sharing a single unit for several channels, but this approach is still expensive and control is cumbersome. A second approach uses the natural error correction property associated with FEC. Since PMD impairments tend to have intrinsic long time constants, an additional external mechanism is introduced to rapidly and continuously perturb optical signal polarization. Such rapid polarization rotation distributes PMD errors more randomly, and makes FEC algorithms more effective in correcting them if randomization occurs within the FEC code block length.
The primary limitation of the present state of the art is the focus on complete randomization of the polarization states. Thus, the performance of each wavelength channel rapidly covers a large fraction of the possible performance states, including ones that induce low penalties and ones that induce high penalties. A channel experiences high penalties for at least a fraction of the FEC block length, and some of the FEC correction capability is thus spent on the PMD problem. Less FEC error correction capability is available for other impairments, such as optical noise, nonlinear effects, linear cross talk effects, etc. What we desire is a mechanism that would still mitigate PMD impairment, while minimizing the amount of FEC capability allocated to this particular impairment.
S. P. Jung, et al. describe the use of in-line polarization controllers between fiber-optic segments to controllably realign polarizations to improve the performance of worst-case PMD channel. S. P. Jung, et al. “Multi-channel PMD compensation based on distributed polarization control,” OFC 2005, vol. 3, March 2005, paper JWA18. The paper suggests the use of dither of polarization control stages with feedback from PMD monitoring of an aggregate sum PMD impairment for all WDM signals. This approach suffers from several limitations, as follows: aggregate PMD monitoring for all WDM signals relying on RF spectrum is not applicable to a number of channels over ˜4, aggregate PMD monitoring is not applicable when channels use different modulation formats or data rates. Further, the feedback loop takes too long to close, as there is intrinsic fiber delay of ˜10 ms associated with a round trip time over 1000 km of a typical fiber link, and the paper does not discuss a mechanism for dealing with highly meshed networks with many optical add/drop multiplexers, where WDM channels may traverse many different overlapping fiber segments.
Thus, present state of the art has numerous performance, cost and control algorithm limitation which the present invention proposes to overcome.